Detection of strabismus in subjects is typically performed manually. Under such circumstances, a trained clinician assesses a subject's eyes using the “Hirschberg test”, which is a well-established, but rough, test for checking if the pupils of the eyes are symmetrical when in focus and fixating on a target placed directly in front of them.
Broadly speaking, the Hirschberg test is carried out by shining a light at the eyes of a subject from a light source positioned directly in front of the eyes, getting the patient to focus on the light or an object right next to it, and seeing whether the distance between the reflection of the light and the centre of the pupil appears to be equal in magnitude and direction in both eyes.
Small differences in each of the eyes of a subject are difficult to determine manually and are therefore easy to miss, even by experienced clinicians. Typically, differences of anything less than about 1 mm are too small to see reliably. Difficulties with the test are exacerbated when the subject is a small child, as it is hard to get a child to fixate and focus on a target for long enough to assess each eye. Therefore, the decision of whether the eyes are symmetrical is often made quickly and without complete information. It is also a test that requires considerable skill, and no is impractical for many testing scenarios, such as testing all pre-school children in a given territory.